Double shoulder threaded connections on oilfield tubulars typically include a pin connector at one end of the tubular and a box connector at the other end. Each connector is adapted to mate with a corresponding connector at the opposite end of another tubular.
The pin connector usually includes a large inside diameter or flow path and external threads extending axially between a radially outward external shoulder and a radially inward pin face. The pin connector also includes a base section extending axially between the external shoulder and the external threads, and a nose section extending axially between the pin face and external threads. The box connector typically includes an inside diameter defining a flow path substantially consistent with that of the pin connector inside diameter, internal threads extending axially between a radially inward internal shoulder and a radially outward box face for threaded connection with the pin connector, and a counterbore section located between the internal threads and the box face.
The external threads and internal threads typically include a taper extending radially outward from a first pin thread adjacent the nose section to a last pin thread adjacent the base section that is sufficiently tapered to allow quick and efficient connection of the pin and box connectors.
In conventional drill pipe, there is usually no internal shoulder in the box member for abutting engagement of a nose or face of the pin. When the pin and box connectors are rotatably connected at the surface, a torque is reached that stresses the pin at the last engaged thread to about one-half its yield strength. If additional torque is imparted during drilling operations, it is possible to exceed the torsional strength of the threads on the pin and box connectors. Consequently, it is advantageous to utilize tool joints with high torque transmission capabilities in order to overcome the weaker threaded connection.
While a number of attempts have been made to create a threaded connection with high torque withstanding abilities, very few have concentrated on the ability to withstand torque in order that the shear stress on the threads is no greater than the strength of the entire tool joint, including the threaded connection. As a result of attempts to withstand--torque in the threaded connection, various design changes have been made to tool joints while attempting to maintain a maximum inside diameter or flow path.
For example, U.S. Pat. No. 4,548,431 to Hall et al presents a tool joint designed to withstand higher torque loading than conventional tool joints. The Hall et al design incorporates a threaded connection having a pin nose section diameter that decreases as the thread length is increased. Thus, since the torsional strength of the Hall et al tool joint is contingent upon the diameter of the pin nose section, increasing the thread length adversely affects the torsional strength. As later determined by testing of the Hall et al design, connections designed with thread lengths adequate for the smallest anticipated inside diameter resulted in a cross-sectional area of the pin nose section at the largest diameter available that was too small. Larger inside diameters produced a pin nose with inadequate strength compared to the pin base, the box counterbore section and the threads. Thus, the threaded connection was not balanced.
U.S. Pat. No. 5,492,375 reveals an improvement over the Hall et al patent. The '375 patent is directed to maximizing the torsional strength of the threaded connection by optimizing the thread length and nose diameter for any given inside diameter. However, neither Hall et al nor the '375 patent strike a geometrically balanced threaded connection without the necessity of correlating the nose diameter or transverse cross-sectional area with the thread length.
Additionally, U.S. Pat. No. 4,549,754 utilizes a thread design that linearly distributes loads along the several threads by decreasing the taper on the external threads relative to the internal threads, such that the taper of the external threads is generally less than the taper of the internal threads.